Which is more important in ballistics?
In a defensive rd for a handgun which is more important to consider....muzzle velocity or muzzle energy?

IMTHDUKE Sets before us two different questions.
First there is the question "Which is more important in ballistics?"
Then there is the far different question "In a defensive rd for a handgun which is more important to consider....muzzle velocity or muzzle energy?"

The first presents a large area to consider because the question is so open ended. It needs to be more specific.

The second question is a bit more specific. At least it focuses on defensive rounds for handguns. This indicates to me that we are not discussing those handgun cartridges either too powerful or too weak to be considered "defensive" handgun cartridges. Also we are not to consider rifle or shotgun cartridges.

I think the second question is what IMTHDUKE meant for us to address.
We must keep in mind what the primary task of a "defensive" handgun cartridge is. The task or goal of a defensive" handgun cartridge is to incapacitate an imminent threat of great bodily harm or death as quickly as possible. To be successful, this cartridge must be fired from a handgun which the shooter will have on their person at the moment the threat is recognized which limits its size and weight. Further the defensive shooter must me able to competently respond to the imminent threat within a time frame that saves the shooter from great bodily harm or death which requires shooter awareness and skill.

As has been stated in this thread, what the projectile must do over 95% of the time to stop a threat is to cause bleeding to such an extent that the threat is incapacitated. Most of the time this bleeding is caused by crushed vascular tissue.

So we are not so much discussing external ballistics as we are anatomy and the destruction caused inside of the body by handgun projectiles which encounter various tissues at various velocities.

So in the context of elastic vascular tissue destruction, constrained within the parameters of "defensive" handgun cartridges; which is more important muzzle velocity or muzzle energy? We are still left with a great deal to consider. Muzzle energy is dependent upon muzzle velocity. One can not stand on its own. Each must be related to other factors such as bullet construction, bullet mass, diameter, sectional density, etc.

An interesting though exercise.

One thing to consider is that incapacitation due to bleeding takes time. This time frame is highly variable.

"When plunder becomes a way of life for a group of men living together in society,
they create for themselves in the course of time a legal system that authorizes it and a moral code that justifies it." Frederic Bastiat